said by ruraltn :

Porting a number is supposed to automatically cancel the old service associated with that number.

said by NetFixer :

I have ported two numbers, and that has not been my experience. In each case, after the port took effect, incoming calls would be sent to the new service, but the account with the original provider remained open (and billed) until I called and canceled the service. YMMV

said by ruraltn :

I stand corrected. On a wire-line to wireless, there may be a duality of service. My experience has been only with wireless to wireless, in which case the old service was lost immediately.

»www.fcc.gov/cgb/consumerfacts/numbport.html

I still recommend that the OP not start the porting process until he has satisfactory dry DSL connectivity, or he may experience an interruption of service.